1. Technical Field
The present invention relates to a method and a device for detecting a thickness of an optical disc, and more particularly to a method and a device for detecting a thickness between respective layers of an optical disc in an optical disc drive.
2. Description of the Related Art
During a process of recording or reproducing an optical disc by an optical disc drive, the thickness of the transparent plastic layer of the optical disc is an important factor influencing occurrence of a spherical aberration effect. In addition, the thickness of the transparent plastic layer of the optical disc is employed to judge the type of the optical disc (e.g. CD, DVD or BD). Generally, the thickness of the transparent plastic layer of the optical disc is equal to the distance between a surface layer and a data layer of the optical disc.
FIG. 1 is a schematic functional block diagram illustrating a device for detecting the thickness of a transparent plastic layer of an optical disc in a conventional optical disc drive. As shown in FIG. 1, the thickness detecting device 100 comprises a processing unit 110, a motor driver 120, a focusing actuator 125, an objective lens 130, a photo sensor 140 and a pre-amplifier 150. The processing unit 110 is a digital signal processor (DSP) for outputting a focus control output signal FCO to the motor driver 120. According to the focus control output signal FCO, the motor driver 120 issues a focus motor output signal FMO to the focusing actuator 125. According to the focus motor output signal FMO, the focusing actuator 125 generates a driving force F to move the objective lens 130. During the objective lens 130 is moved, the light beams B reflected by the optical disc are sent to the photo sensor 140. As such, the change of the light intensity is detected by the photo sensor 140. When the focus point of the light beams is moving, plural photo signals PS are generated and transmitted to the pre-amplifier 150. By the pre-amplifier 150, the photo signals are synthesized into a beam intensity signal BS and a focusing error signal FE, which are transmitted to the processing unit 110.
Conventionally, for detecting the thickness of the transparent plastic layer of the optical disc, the focus control output signal FCO issued by the processing unit 110 is gradually increased. The focus control output signal FCO is amplified into the focus motor output signal FMO by the motor driver 120. According to the focus motor output signal FMO, the focusing actuator 125 controls movement of the objective lens 130 toward the optical disc.
FIG. 2 is a schematic diagram illustrating associated signals processed in the thickness detecting device of FIG. 1. As the objective lens 130 is ascended, focus point of the laser beam successively cross the surface layer of the optical disc. In this situation, the beam intensity signal BS has a first peak value with lower amplitude (also referred as a surface layer signal). As the objective lens 130 is continuously ascended and the focus point reaches the data layer, the beam intensity signal BS has a second peak value with higher amplitude (also referred as a data layer signal). By measuring the time interval T between these two peak values, the thickness between the surface layer and the data layer of the optical disc is deduced.
Alternatively, as the objective lens 130 is ascended, focus point of the laser beam successively cross the surface layer of the optical disc. In this situation, a first S curve of the focusing error signal FE having a lower peak-to-peak value is obtained. As the objective lens 130 is continuously ascended and the focus point reaches the data layer, a second S curve of the focusing error signal FE having a higher peak-to-peak value is obtained. By measuring a time interval between the zero crossing points of the first S curve and the second S curve, the thickness of the transparent plastic layer of the optical disc is deduced.
In other word, once the focus control output signal (FCO) outputted from the processing unit 110 is multiplied by the gain value of the motor driver 120, the speed (v) of moving the objective lens 130 is obtained. The thickness of the transparent plastic layer of the optical disc is easily obtained according to the formula: Δd=v×T.
However, due to the mass production of the optical disc drives, the gain values of the motor driver 120 and the focusing actuator 125 are usually suffered from variations. That is, even if the settings of the motor drivers 120 of different optical disc drives are identical, the focus motor output signals outputted from the motor drivers 120 are different. As such, the thickness Δd of the transparent plastic layer of the optical disc by the conventional method usually results in large error. Under this circumstance, erroneous judgment of the thickness occurs.
That is, if the gain value of the motor driver 120 or the focusing actuator 125 is inconsistent because the producing quality of the thickness detecting device 100 is deteriorated, the thickness of the transparent plastic layer of the optical disc is not believable.
FIG. 3 is a schematic diagram illustrating associated signals processed in the thickness detecting device, in which the gain value of the motor driver 120 is varied. Assuming that two optical disc drives issue identical focus control output signal FCO and the settings of the gain values are identical, the objective lenses of the two optical disc drives are both moved in the speed v. Since the gain values of the motor driver 120 and the focusing actuator 125 are varied, the focus motor output signals are distinguished. As shown in FIG. 3, when an optical disc is loaded in a first optical disc drive having a lower gain value than a second optical disc drive, the slop of the first focus control output signal FCO1 from the first optical disc drive is lower than the slop of the second focus control output signal FCO2 from the second optical disc drive. As such, a first beam intensity signal BS1 indicated as the solid line is obtained by the first optical disc drive, and a second beam intensity signal BS2 indicated as the dotted line is obtained by the second optical disc drive. The time interval T1 between the two peak values of the first beam intensity signal BS1 is longer than the time interval T2 between the two peak values of the second beam intensity signal BS2 (i.e. T1>T2). The thickness of the transparent plastic layer of the optical disc is obtained by the first optical disc drive according to the formula: Δd1=v×T1. The thickness of the transparent plastic layer of the optical disc is obtained by the second optical disc drive according to the formula: Δd2=v×T2. The thickness difference between these two optical disc drives is about 20% or even 50%.
Similar, the method for detecting the thickness of the transparent plastic layer of the optical disc according to the focusing error signal FE may result in thickness difference when two optical disc drives are used. In other words, the conventional thickness detecting method is not suitable to detect the thickness of the transparent plastic layer of the optical disc if the gain values of different optical disc drives are different.